The present invention relates generally to photolithography techniques in semiconductor manufacturing and, more particularly, to a method and system for phase/amplitude error detection for alternating phase shifting masks in photolithography.
Lithography in the context of VLSI/LSI manufacturing of semiconductor devices refers to the process of patterning openings in photosensitive polymers (sometimes referred to as photoresists or resists) that define small areas in which a silicon base material is modified by a specific operation in a sequence of processing steps. The manufacturing of semiconductor devices chips involves the repeated patterning of photoresist, followed by an etch, implant, deposition, or other such operation, and ending with the removal of the expended photoresist to make way for the new resist to be applied for another iteration of this process sequence.
A basic lithography system includes a light source, a stencil or photo mask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. The aligning may take place in an aligning step or steps, and may be carried out with an aligning apparatus. Since a wafer containing from 50 to 100 chips is patterned in steps of 1 to 4 chips at a time, these lithography tools are commonly referred to as steppers. The resolution, R, of an optical projection system such as a lithography stepper is limited by parameters described in Rayleigh's equation:R=kλ/NAwherein λ represents the wavelength of the light source used in the projection system. NA represents the numerical aperture of the projection optics used, and “k” represents a factor describing how well a combined lithography system can utilize the theoretical resolution limit in practice.
Conventional photo masks typically include opaque chromium patterns formed on a quartz plate, thereby allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask and onto the photoresist coated wafer, exposing the resist wherever corresponding hole patterns are placed on the mask. Exposing the resist to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers, which allow a developer to dissolve and remove the resist in the exposed areas. (In contrast, negative resist systems allow only unexposed resist to be developed away.)
The photo masks, when illuminated, can be pictured as an array of individual, infinitely small light sources which can be either turned on (points in clear areas) or turned off (points covered by chrome). If the amplitude of the electric field vector which describes the light radiated by these individual light sources is mapped across a cross section of the mask, a step function will be plotted reflecting the two possible states that each point on the mask can be found (light on, light off).
These conventional photo masks are commonly referred to as chrome on glass (COG) binary masks, due to the binary nature of the image amplitude. However, a perfectly square step function of the light amplitude exists only in the theoretical limit of the exact mask plane. At any given distance away from the mask, such as in the wafer plane, diffraction effects will cause images to exhibit a finite image slope. At small dimensions (i.e., when the size and spacing of the images to be printed are small relative to λ/NA) the electric field vectors of adjacent images will interact and add constructively. This is due to the wave nature of the radiation, in which it spreads as it propagates. As a result of this diffraction effect, the light intensity curve between the features is not completely dark, but exhibits significant amounts of light intensity created by the interaction of adjacent features. Because the resolution of an exposure system is limited by the contrast of the projected image, an increase in the light intensity in nominally dark regions will eventually cause adjacent features to print as one combined structure rather than discrete images.
The quality with which small images can be replicated in lithography depends largely on the available process latitude (i.e., the amount of allowable dose and focus variation that still results in correct image size). Phase shifted mask (PSM) lithography improves the lithographic process latitude or allows operation at a lower “k” value by introducing a third parameter on the mask. The electric field vector, like any vector quantity, has a magnitude and direction. As such, in addition to turning the electric field amplitude on and off, it can be turned on with a phase of about 0° or turned on with a phase of about 180°. This phase variation is achieved in PSMs by modifying the length that a light beam travels through the mask material. By recessing the mask to an appropriate depth, light traversing the thinner portion of the mask and light traversing the thicker portion of the masks will be 180° out of phase. In other words, their electric field vector will be of equal magnitude but point in exactly opposite directions so that any interaction between these light beams results in perfect cancellation.
Although the use of alternating phase shift masks has advantages such as improved resolution, larger exposure latitude and larger depth of focus, it can also generate overlay errors if there are phase errors or intensity transmission errors between the neighboring mark openings. For example, if the etch depth of a phase shifted opening in the quartz is incorrect, there will be a phase error associated therewith and the diffraction interference will not be completely destructive at the original symmetry axis intersecting the position of the line perpendicular to the mask plane. The position of the minimum will thus be shifted laterally in the image plane (the degree of shift being proportional to the amount of defocus), thereby causing overlay error on the wafer. In addition to phase error, there could also be an amplitude mismatch in light transmission through the two adjacent, out-of-phase openings. If this error exists, the space CD printed on the wafer will be unequal, even at best focus.
While there are certain techniques in existence that detect phase defects, they typically involve either the direct measurement of etch recess using physical probes or the combined measurements of both the asymmetry in aerial image and computer simulation. The former can be accurate in measuring the etch depths and mask dimensions, but it does not provide the data for intensity mismatch since it also depends on the light scattering characteristics, and it is not a direct measurement of phase cancellation. The latter is capable of measuring both intensity mismatch and phase error but, due to the use of computer simulation, the accuracy of the results can be subject to various approximation errors. In addition, the precise measurement of the amplitude error is difficult since it is highly dependent on the location of the best focus, which is difficult to obtain within 100 nm (1×). Accordingly, it is desirable to be able to accurately detect phase and amplitude defects independently.